A primary challenge with processing and storing most polymers, especially polyolefinic polymers, styrenic polymers, and poly(meth)acrylate polymers, is the susceptibility of the polymer to undergo oxidative degradation. Polymeric compounds, for example polyolefins like polyethylene and polypropylene, undergo radical driven degradation processes especially during elevated temperature processing steps which might include moulding, extrusion etc. For example, during melt-extrusion, the rate of oxidation of melted polymeric materials gradually increases as the polymeric materials are brought to their melting temperature. The polymeric materials degenerate in the presence of the ambient oxygen to low molecular weight gels, discolored condensates and the like. The origin of the initiating radical species of the degradation process is not completely understood, but under heat processing, peroxide radicals are formed by reaction with molecular oxygen. The peroxide radicals in turn create alkyl radicals by abstracting hydrogen radicals from the polymer backbone, which leads to cross-linking and chain scission. However, degradation even proceeds during end-use by a radical mechanism under the influence of light, heat etc. and will finally destroy the polymer properties.
There are many methods described in the prior art that address stabilization of polymers during processing to alleviate the effects of heat, shear, and degradation of the polymer architecture. A wide variety of chemical additive claims have been made, which typically call out a common formulae to include at least a phenolic antioxidant, a phosphorous based stabilizers, and an antacid. Additionally, the prior art also teaches compaction and extrusion techniques to convert these common formulae of powder materials into non-dusting physical forms which can improve the chemical hygiene of handling the materials. For example, EP 0565184 describes a process for obtaining granular forms from mixtures of powders of two or more additives for organic polymers by extruding the mixture at a temperature between the melting point of the component with the lowest melting point and 140° C. And U.S. Pat. No. 6,143,814 describes a fusible stabilizer composition that is produced by a method in which at least one metal carboxylate is produced in situ from a corresponding carboxylic acid melt and an at most stoichiometric quantity of metal oxide, hydroxide, carbonate, and/or basic metal carbonate, wherein the carboxylate is held in the melt, until further fusible or softenable components are then added with stirring, and then all non-fusible components are added. However, both of these references disclose the use of phosphorus based stabilizers and are silent with respect to minimizing their presence.
However, there numerous deficiencies caused by the use of the phosphorous based stabilizers (e.g., phosphite and phosphonite compounds). For example, phosphites are known to hydrolyze, leaving behind black specs in the polymer and contributing to discoloration.
Moreover, many studies have been performed on the physical parameters of phosphorus based stabilizers, which include diffusion coefficient in polymer and equilibrium solubility. The most common commercial phosphite[tris(2,4-di-t-butylphenyl)-phosphite] (CAS #31570-04-4), has very low solubility in polyolefins which leads to a phenomenon called blooming. Blooming of the phosphite based stabilizer causes the material to plate out on equipment and remain on the surface of the polymer after processing, thus requiring treatment.
There are critical parameters when food and medical applications are considered. Migration of the additives used for stabilization of the polymer during processing must be suitable for these uses, but more importantly, because the additives will undergo chemical reaction during processing it is imperative that the by-products of the stabilization additives are not harmful. One case where use of phosphite type stabilizer has come under scrutiny is very commonly used phosphite, trisnonylphenol phosphite (TNPP), (CAS #26523-78-4). Although TNPP has limited environmental and human health concerns, its hydrolysis product yields nonylphenol which is under scrutiny by the U.S. Environmental Protection Agency (see e.g., U.S. EPA Nonylpheonl (NP) and NonylphenolEthoxylates (NPEs) Action Plan, RIN 2070-ZA09, Aug. 18, 2010). Moreover, because the environmental and human health issues are a global concern, the use of TNPP has been regulated out of polymers in some countries. While free nonylphenol content may be negligible in commercially available TNPP products, the hydrolysis of TNPP yielding nonylphenol presents concerns with its use and as such the industry is seeking alternatives.
Unfortunately, many of the commercially available phosphorous based antioxidants, e.g., bis(2,4-di-t-butylphenyl)pentaerythritol diphosphite, bis(2,4-dicumylphenyl)pentaerythritol diphosphite, distearyl pentaerythritol diphosphite, (CAS #87498-44-0, CAS #154862-43-8, CAS #38613-77-3, CAS 119345-01-6, CAS #3806-34-6), also suffer from similar deficiencies.
Therefore, a need exists for new methods of stabilizing polymers, which can decrease or eliminate the need for phosphorus based stabilizers.